Complex Ecosystems Better Resist Extinction ‘Cascades’

Like a game of biological Jenga, the disappearance of a species could bring down an entire ecosystem, wiping out other animals around them.

But the more species a community holds, the less likely it is to suffer from a “cascade” of secondary extinctions when one species dies out, British researchers report. Their study could help other scientists understand the ongoing wave of extinctions rippling through the Earth’s environment.

“Each species has a certain function,” Dirk Sanders, a community ecologist at the University of Exeter, told Seeker. If you remove a species, like the game in which players remove wooden blocks from a tower, “you lose that function. That means we get knock-on effects throughout the whole system, and they’re not necessarily obvious.”

Sanders and his colleagues tested their theory by setting up their own communities at a research station in southwest England. They populated those 40 micro-environments with differing mixes of bean and barley plants; three species of plant-eating aphids; three or four species of parasitic wasps that lay their eggs in the aphids; and eight species of hyperparasitic wasps, which live off the parasitic wasps. Some communities were more complex than others.

Then they started pulling a block out of that ecological Jenga tower by targeting one of the parasitic wasp species. Over the 14-week study, the researchers started plucking out the host aphid “mummies” that the parasites’ larvae turned into cocoons, cutting the population of those wasps by roughly half.

As might be expected, the population of the aphid species the wasps had used as hosts grew — but other aphid species that weren’t directly disturbed found themselves facing more competition, so their numbers plunged. So did the number of parasitic wasps that laid eggs in those species.

“For the target species, it was not necessarily a problem,” Sanders said. “But the effects for the whole system were actually quite dramatic.”

Meanwhile, in more complex communities that included other species of parasitic wasps, the aphid numbers stayed relatively stable, with fewer extinctions.

“The first ones to go extinct are not directly linked,” Sanders said. “It all happens through the herbivore level, where these aphids interact and they compete for the resource.”

The findings were published February 20 in the research journal Proceedings of the National Academy of Sciences. Sanders, the study’s lead author, said earlier studies were limited to computer models that simulated the expected effects of a species disappearing.

The study comes as the world is experiencing what some scientists are calling a sixth mass extinction period. Despite recent efforts at conservation, species are disappearing 1,000 times faster than they have in the past centuries. Most of that is the result of human activity — destruction of habitat, hunting and fishing, pollution, and climate change.

Kristy Bly, a senior wildlife biologist with the World Wildlife Fund, told Seeker the study adds “concrete evidence” to what many researchers have long suspected, but have found difficult to measure.

“In my experience on the prairie, we see this almost daily with the loss of the prairie dog, which is a keystone species of the grassland ecosystem on the northern Great Plains,” said Bly, who’s based in Montana. For example, up to 150 different species depend in some way on the black-tailed prairie dog, including the black-footed ferret — a prairie dog predator listed as an endangered species. But the prairie dogs have lost 95 percent of their original habitat, and the animals that remain have been ravaged by outbreaks of a form of bubonic plague.

“It’s really hard to impact one thing without having the rest of the system kind of collapse,” she said. But she added that while it’s great to have a scientific demonstration of the effect, “What are we going to do about it?”

“We have a responsibility to make sure that much like city planning, you’re thinking about the consequences of these actions before we get to the point where it’s irreversible,” Bly said. While environmentalists have succeeded in bringing some species back from the brink of extinction, she said, species not only need to eke out a living but mingle with different populations to thrive.

“Money is so limited for species we don’t eat or can otherwise make money off of,” she said. “We need to understand that populations, in order to thrive, must be connected.”

Sanders said the study can help explain the “dramatic loss of biodiversity” now being seen in some places.

“If you have a very complex system, the whole thing is very tight,” he said. “You just take a few of these blocks out, and nothing really happens. But if you’ve taken a lot out already, it’s more likely that something really bad happens.”